1. Field of the Invention
The present invention relates to an information recording medium in the form of a disc for writing or reading information by means of laser beam of high energy density.
2. Description of Prior Arts
Information recording media utilizing a beam of high energy density such as laser beam have been developed in recent years and are put into practical use. The recording medium is called an optical disc and the practical applications thereof have been found, for example, as video disc and audio disc as well as disc memory for large-capacity computer and large-capacity static image file.
The optical disc basically comprises a disc-shaped transparent substrate of a plastic or glass material and a recording layer composed of a metal such as Bi, Sn, In or Te or a semi-metal provided on said substrate. On the surface of the substrate where the recording layer is to be coated is generally provided an additional layer such as an undercoating layer or an intermediate layer for improving smoothness of the surface of the substrate, and for increasing adhesion to the recording layer and sensitivity of the resulting disc. Writing of information on the optical disc can be conducted, for example, by irradiating the optical disc with laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the optical disc absorbs energy of the beam, rise in temperature locally occurs, and as a result chemical or physical change is caused to alter optical characteristics of the recording layer in the irradiated area, whereby the recording of information can be made. Reading of information from the optical disc is also conducted by irradiating the optical disc with laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the alteration of the optical characteristics of the recording layer.
As disc structures for protecting the recording layer, there have been proposed two sheet structures. One is a directly-combined structure in which a recording layer is provided on at least one of two disc-shaped substrates and positioned inside the two substrates opposed to each other, the two substrates being combined directly with each other on their peripheries by means of an adhesive. Another is an air-sandwich structure in which a recording layer is provided on at least one of two disc-shaped substrates and positioned inside the two substrates opposed to each other at a certain distance, the two substrates being combined with each other via a ring-shaped inner spacer and a ring-shaped outer spacer by an adhesive so as to form a closed space surrounded by the two substrates and two spacers. In the optical disc having such structure, the recording layer is not directly exposed to outer air, and the recording and reproduction of information can be conducted with laser beam passing through the transparent substrate. Accordingly, there is little possibility that the recording layer is physically or chemically damaged or the surface is covered with dust which is to interfere with the recording and reproduction of information of the recording medium.
However, the information recording disc having the above-mentioned directly-combined structure or air-sandwich structure has insufficient mechanical strength in the region (i.e., area) combined by the adhesive, whereby the disc is easily damaged, as compared with a recording disc of simple structure comprising one substrate and a recording layer provided thereon. In more detail, in certain cases, stress is produced in the disc having such structure due to expansion or contraction of the substrate depending on the conveying conditions or storage conditions such as temperature or humidity. Further, particularly in the case of air-sandwich structure, stress is also produced by expansion of air in the closed space. In these cases, the stress tends to concentrate on the adhesive layer (i.e., adhesive-combined portion of the disc), and hence the substrate is liable to separate from each other or from the spacer. In addition to the disadvantage, it is required for the disc of air-sandwich structure to select such adhesive as not producing deterioration of the recording layer through diffusion of decomposition products of the adhesive. There are some cases that a suitable adhesive having satisfactory adhesion for the employed substrate is hardly found out.
Furthermore, in some cases, even if the recording disc is not damaged, the combined region of the disc is deteriorated in sealing property to allow outer air containing water vapor to pass into the disc, whereby the sensitivity of the recording disc decreases.